The server principal “user” is not able to access the database “db” under the current security context.

I was running into an issue that I really needed to resolve quickly.
I was getting the below error:

The server principal "sqluser" is not able to access the database "db" under the current security context. (Microsoft SQL Server, Error: 916)

After googling a lot I found a blog : https://connect.microsoft.com/SQLServer/feedback/ViewFeedback.aspx?FeedbackID=354291.

Resolution:
This suggestion solved my problem:
Please try using the following workaround and let us know if that helps:

1) Bring Object Explorer Details window by selecting View --> Object Explorer Details in menu (or hitting F7)
2) In Object Explorer window click at Databases folder
3) In Object Explorer Details window right-click at the column header and deselect Collation
4) Refresh Databases folder.

Solved.

Regards,
Taruna